Most of the seismic lines of Cruise M63/1 were acquired in the area off the Zambezi River (Fig.
1.7). Sediment structures were most clearly influenced by terrigenous sediment input, which had been the main objective of survey and sampling activity during the cruise.
Line GeoB05-005 (Fig. 1.8) is located at the continental slope south of the Zambezi Fan (Fig.
1.7). The seismic data were acquired in water depths of 1200 to 1600 m. Seafloor topography is very rough and, especially the NNW part, frequently interrupted by V-shaped depressions. Some transparent lenses at CMP 1200 to CMP 1900, intercalated between stronger reflectors, probably indicate the presence of slump deposits. The sediment body in the southeastern part reveals in the upper 180 ms TWT layers of high signal energy. Between CMP`s 300 and 800, a buried V-shaped sediment body with complex lateral and vertical structures may indicate refilling of an old trough or channel. Another channel-like structure or trough appears at the surface between CMP`s 300-500. A pronounced blanking zone at CMP 1500 masks deeper reflections beneath 2000 ms TWT.
METEOR-Berichte 09-3, Cruise 63, Leg 1, Cape Town – Cape Town 1-16
Fig. 1.7: Track chart and seismic profiles in Area C (Zambezi Fan).
Fig. 1.8: Multichannel seismic profile GeoB05-005 from southern part of the Zambezi Fan, dominated by pronounced slump deposits and disturbed sediment layers.
The PARASOUND data of Line GeoB05-008 are shown in Figure 1.9. It is 160 km long and oriented from SSE to NNW, located east of the Zambezi estuary mouth. The PARASOUND profile starts in a water depth of 2000 m and shows a gentle northward ascend of the sea floor up to a water depth of 100 m. The penetration depth of the PARASOUND signal varies from 10 m near the shelf break, to 40 m in the central part of the profile with water depths between 800-1300 m.
Numerous incisions of variable dimensions up to 100 m depth are noticeable features in this central section down to 1800 m water depth. This slope section was also a target for sampling by gravity corer. The closeup of core station GeoB 9309 (Fig. 1.9) shows for this position a well stratified sediment package of 45 m thickness. The deposits show few stronger reflectors inter-calated with weaker reflectors or rather transparent layers.
METEOR-Berichte 09-3, Cruise 63, Leg 1, Cape Town – Cape Town 1-18
Fig. 1.9: PARASOUND image of multichannel seismic profile GeoB05-005 from eastern part of the Zambezi Fan.
Multichannel seismic Line GeoB05-010 (Fig. 1.10) reveals the main sedimentary structures off the mouth of the Zambezi estuary (Fig. 1.7) close to the shelf edge. From shallow water down to 900 m (1200 ms at CMP 800), seafloor topography is smooth. At greater water depth several smaller incisions occur at the sea floor. Towards the southwest, a near-surface zone of decreased reflection amplitudes reveals as a lens-shaped sediment body with a relatively sharp transition to a zone of stronger reflectors at CMP 950. This sigmoidal sediment packet extends over a dis-tance of about 50 km between CMP 950 and 2100. The greatest thickness with 140 m at CMP 1740 occurs at the uppermost slope in water depths of about 340 m (400 ms TWT). Downslope the thickness of this sediment body decreases gradually.
Figure 1.11 shows the PARASOUND image of multichannel seismic Line GeoB05-010 from CMP 1000 up to the shelf break over a distance of 37 km in NE-SW direction. The penetration depth of the PARASOUND signal ranges from a few meters on the shelf to about 50 m in water depths deeper than 500 m. The sea floor topography in this section is very smooth. Only some smaller incisions occur in the uppermost part in water depths between 100 m and 170 m. This section of the continental slope was also a target for gravity core sampling. The coring station GeoB 9310 is located in a water depth of about 550 m (Fig. 1.11). The close up shows the sea floor as a stronger reflector with a more transparent zone beneath.
Fig. 1.10: Multichannel seismic profile GeoB05-010 in the vicinity of the shelf break, directly in front of the Zambezi estuary mouth.
METEOR-Berichte 09-3, Cruise 63, Leg 1, Cape Town – Cape Town 1-20
Fig. 1.11: PARASOUND image of multichannel seismic profile GeoB05-010.
A second short reflector segment of high signal energy occurs in the lower left corner of the closeup. This reflector terminates after 3 km in both directions. Between these two layers, weaker reflectors or almost transparent layers are intercalated. In Figure 1.10 of multichannel seismic Line GeoB05-010, CMP 1500 defines the core location. This figure indicates the position of station GeoB 9310 more ore less in the centre of the sigmoidal sediment package, which, how-ever, does not appear at all in the PARASOUND image, but is rather identified as an area of highest accumulation rate in the seismic line.
1.4.3 Sedimentology
(U. Bleil, K. Enneking, Chr. Hilgenfeldt, S. Kasten, M. Klann, H. Kuhlmann, A. Lückge, Chr. März, E. Schefuß, R. Schneider, A. Steinbach, E. Susek, S. Weldeab, N. Zatloukal) 1.4.3.1 Sediment Sampling with Gravity Corer
During cruise M63/1 24 sediment cores on 14 stations were recovered using the gravity corer SL-18, SL-12, SL-9, and SL-6 (Table 1.2 and Figures 1.12 and 1.13). At 9 of these stations addi-tional sediment cores were taken for geochemistry purposes (Table 1.4, Chapter 1.4.5). Once the core was retrieved on the deck, the core liners were cut into 1 m segments, closed with caps at both ends and inscribed according to the scheme applied at the Geosciences Department, University Bremen.
All cores were cut along-core in two half pieces: one archive and one work-half. The sedi-ments were described and photos were taking. For color scanning, a MINOLTA 2006d hand-held spectrophotometer was used to measure percent reflectance values of sediment color over the visible light range between 400 nm and 700 nm. The digital reflectance data of the spectro-photometer readings were routinely obtained from the surfaces of split archive halves imme-diately after the core opening to provide a continuous record of the sediment color variation.
From the work-half three parallel series of syringe samples (10 cm3) were taken at intervals of 5 cm. These samples were taken for the measurements and determination of stable isotopes, foraminiferal assemblages, organic geochemistry, and trace element analyses on foraminiferal tests.
The preliminary lithologic summary of the sediments retrieved with gravity corer is based on visual description and color scanner data. A compilation of color scans from selected gravity cores is shown in Figs. 1.14, 1.15, and 1.16). Core descriptions are published in the extended version of this cruise report in the « BERICHTE aus dem Fachbereich Geowissenschaften der Universität Bremen, Report and Preliminary Results of Meteor Cruise M63/1, 2008 ». There, figures representing the main lithologies, their color according to the MUNSELL soil color chart, and the sedimentary structure as well as color scanner readings of the ratio 700 nm/400 nm (red/blue ratio) and L* of the core sediments are also presented.
Tab. 1.2: List of gravity cores (SL) retrieved during M63/1.
GeoB
METEOR-Berichte 09-3, Cruise 63, Leg 1, Cape Town – Cape Town 1-22
Limpopo River
Maputo
29°S 28°S 27°S 26°S
36°E 34°E 35°E
32°E 33°E
36°E 34°E 35°E
32°E 33°E
29°S 28°S 27°S 26°S
Tugela River
30°S 30°S
25°S 25°S
Maputo River Sabie River
-2000
-3000
0 100 200
Km
-2800 -3000
Cruis e Tra c k S ta tion G e oB #
9301 9302
9312
9313
9314
Fig. 1.12: Sites of gravity cores collected off the Tugela River and Limpopo River during M63/1.
Beira
23°S 22°S 21°S 20°S 19°S 18°S
23°S 22°S 21°S 20°S 19°S
35°E 36°E 37°E 38°E 39°E
Zambezi River 18°S
Pungue River
Save River
Luala River
0 100 200
Km
9303
9304 9305
9306 9307
9308
9309 9310
9311
S ta tion G e oB #
Fig. 1.13: Sites of gravity cores collected off the Zambezi River during M63/1.
METEOR-Berichte 09-3, Cruise 63, Leg 1, Cape Town – Cape Town 1-24
Fig. 1.14: Lightness (L*) of sediment cores from the Limpopo Fan (GeoB 9301-6 and 9302-3) and the Sambesi Fan (GeoB9303-2 and 9304-2).
Fig. 1.15: Lightness (L*) of sediment cores from the Sambesi Fan (GeoB 9307-3, 9308-2, 9308-6, and 9310-4).
Fig. 1.16: Lightness (L*) of sediment cores from the Sambesi Fan (GeoB9307-3, 9308-2, 9308-6, and 9310-4).